1. Field of the Invention
This invention relates generally to material handling equipment and more particularly to devices for transferring workpieces into and out of sealable chambers of processing equipment.
2. Description of the Prior Art
The concept and utility of loadlocks, although often referred to by other names such as airlocks, have been recognized for many years. Loadlocks are routinely employed in a wide variety of endeavors whenever an object is to be transferred from an environment within one chamber to a different environment outside that chamber and interaction between the two environments is to be minimized or prevented. An essential structural feature of a loadlock is that it includes a chamber intermediate the two environments to be isolated and means for opening and then closing that chamber first to the space having one environment and then to the space having the other environment. In many applications, the intermediate chamber is brought into pressure and environmental equality with the adjoining space before being opened thereto. A familiar example of such an apparatus is the torpedo tube of a submarine. This device permits a projectile, originally located within the substantially dry and air-filled environment within a vessel, to enter the liquid environment surrounding the hull while minimizing the entry of water into the submarine's interior. Similarly, the airlock of a space vehicle allows its occupants to leave the pressurized environment of its cabin and enter into the vacuum environment of space without exposing the whole interior of the spacecraft to that vacuum environment.
Because of their usefulness, loadlocks are routinely employed in manufacturing equipment to transfer a workpiece into or out of processing chambers while minimizing the entry of other material thereinto or its escape therefrom. In particular, loadlocks are conventionally incorporated into vacuum processing equipment of the type used in the semiconductor fabrication industry. The use of such vacuum processing equipment in this industry is increasing because the processes of sputtering, plasma etching, and reactive ion etching are currently being adopted as replacements for wet chemical processing. The use of loadlocks in these processing systems may be absolutely mandatory in some cases because opening a reaction chamber permits gasses to be absorbed from the air onto its interior surfaces. Frequently, processing performed in a chamber after it has been exposed to air produces results differing greatly from those obtained prior to such exposure. These changes in performance may often be traced to absorbed gasses or, in particular, moisture on the chamber walls which have not been removed by subsequent evacuations. Particularly with plasma or reactive ion etching, opening a chamber can greatly affect its performance because the presence of such absorbed gasses can alter the chemical reactions occurring within it.
Another significant advantage generally obtained by using a loadlock to transfer a workpiece into an evacuated processing chamber is reduced vacuum pump load. Vacuum pump load is reduced because the loadlock usually encloses a much smaller volume than that of the processing chamber. Thus, using a loadlock reduces the volume of gasses to be pumped from the system for each work piece processed in comparison with a corresponding system not employing a loadlock. This reduced pumping load either allows the workpiece to be exposed to a vacuum environment more quickly or permits a reduction in the capacity of the pump employed to establish the vacuum, or a combination of both faster evacuation and reduced pump size.
While the loadlocks currently employed in semiconductor industry processing equipment have common features, e.g. they are generally adapted to handle disk-shaped workpieces having a thickness in the range of 0.010 to 0.050 inches and diameters measured in inches, thus far most manufacturers of vacuum processing equipment have integrated the components of the loadlock into the total structure of the processing system. The absence of a standardized modular apparatus to perform this common function creates several difficulties. First, since the workpiece to be transferred through such a loadlock is fragile and easily contaminated, each manufacturer must expend a significant effort in developing a workable apparatus and making its operation reliable. Furthermore, the magnitude of this development effort is increasing because the trend is to operate vacuum processing systems under computer control with all wafer handling performed automatically without human intervention. A second difficulty is the maintenance and repair of processing equipment in a factory equipped with a variety of systems each having a unique loadlock structure. Such a mixture of equipment requires that those responsible for its maintenance be familiar with the structure and operation of each individual mechanism. Furthermore, such a mixture of equipment may require the maintenance of an inventory of spare parts for each type of loadlock. Thus, it is evident that identifiable benefits could accrue to both those building and those using vacuum processing systems if a standardized, modular loadlock were employed.